In a conventional wireless packet communication apparatus, a wireless channel to be used is determined in advance. Prior to transmission of data packets, carrier sense is performed to detect whether or not that wireless channel is idle. Only when that wireless channel is idle, one data packet is transmitted. This management process enables a plurality of STAs to share one wireless channel in a staggered manner ((1) International Standard ISO/IEC 8802-11 ANSI/IEEE Std 802.11, 1999 edition, Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Phyusical Layer (phy) specifications”, (2) “Low-powered Data Communication System/Broadband Mobile Access Communication System (CSMA) Standard”, ARIB STD-T71 version 1.0, Association of Radio Industries and Businesses, settled in 2000).
On the other hand, a wireless packet communication method is studied in order to improve transmission efficiency of data packets, in which a plurality of data packets are transmitted simultaneously on one wireless channel by using a known MIMO technique (Kurosaki et al., “100 Mbit/s SDM-COFDM over MIMO Channel for Broadband Mobile Communications”, Technical Reports of the Institute of Electronics, Information and Communication Engineers, A. P 2001-96, RCS2001-135(2001-10)). In the space division multiplexing (SDM), different data packets are transmitted from a plurality of antennas on the same wireless channel at the same time. The data packets transmitted at the same time on the same wireless channel are received by digital signal processing that can deal with the difference in propagation coefficients of the respective data packets received by a plurality of wireless antennas of an opposed STA.
FIG. 14 shows a relationship between a transmitting signal and a receiving signal in MIMO. The relationship between the transmitting signal and the receiving signal is represented by a determinant shown in FIG. 14. Propagation coefficients hxx are unknown on a receive side. Thus, the receive side estimates those propagation coefficients, obtains an inversion matrix of a transmission coefficient containing the propagation coefficients, and calculates transmitting values s1, s2, and s3 from the obtained inversion matrix and receiving values r1, r2, and r3.
In general, the propagation coefficients hxx are changed with time and are also changed by a change in a wireless channel such as fading, reduction in signal intensities, and the like. Moreover, when MIMO number is increased, an effect of the change in the wireless channel on the channel condition becomes large. That is, a packet error rate or a bit error rate becomes larger with the increase of the MIMO number. Therefore, the MIMO number is determined (limited) in accordance with the propagation coefficients and the like.
When transmission of a data packet is unsuccessful, the receive side transmits a response packet indicating that failure or does not transmit any response packet. In this case, the transmitting side determines that transmission of the data packet is unsuccessful, and retransmits the data packet. However, retransmission of data packets simultaneously transmitted using MIMO is not specifically defined. Thus, a problem in the case where a conventional retransmission process is applied to such simultaneous transmission is now described.
FIG. 15 shows a general processing on exchanging data packets. After a transmit-side STA transmits a data packet, a receive-side STA transmits an acknowledgement (hereinafter, ACK) packet for the received data packet, thereby giving notice of information about the ratio of successful receptions of data packets to total receptions in the past on the receive-side STA. That method for transmitting an ACK packet can be applied without change to a wireless packet communication method that uses MIMO. In this case, it is considered that a packet exchange sequence as shown in FIG. 16 is performed.
An STA receiving a plurality of data packets multiplexed by MIMO generates ACK packets. The number of those ACK packets is the same as the number of data packets that are successfully received. The thus generated ACK packets are sent back to an STA that is a sender of the data packets while being multiplexed by MIMO. As the number of the data packets successfully received increases, the number of the ACK packets multiplexed by MIMO also increases. As a result, a ratio of successful ACK packet receptions to total receptions becomes lower with the increase of data packets successfully received. Thus, an effect of improving throughput achieved by transmission of data packets using MIMO is weakened.
This is because the transmit-side STA cannot distinguish failure in receiving of the data packets on the receive-side STA from failure in receiving of the ACK packets sent from the receive-side STA on the transmit-side STA. Thus, when the transmit-side STA does not receive the ACK packet, the transmit-side STA determines that transmission of the data packet is unsuccessful and retransmits the data packet. Therefore, in the case where the ratio of successful ACK packet receptions to total receptions is low, it is highly likely that the transmitting side wrongly determines that transmission of the data packet is unsuccessful although the receive side successfully receives the data packet. As a result, unnecessary control, i.e., transmission of the data packet that does not require retransmission is performed, thus reducing usability of a wireless channel.
It is an object of the present invention to, in the case where a transmit-side STA transmits a plurality of data packets simultaneously by using MIMO, surely transmit a packet containing information about the ratio of successful receptions of data packets and total receptions in the past from a receive side, thereby achieving high throughput.